Biomass

Biomass; all earths living matter, is a very abundant source of energy. From
the very beginning of human history the predominant sources for heat and power
were wood for heat and cooking, charcoal (from wood) wind and water for the
power industry and crops. Wood and charcoal are forms of biomass. In an energy
context it can be defined as “all non-fossil organic materials that
have intrinsic chemical energy content”[1].
This energy content is known as Bioenergy. Bioenergy can be
produced from various different biomass resources (or feedstocks) through
various processes making biomass one of the most versatile energy sources.
In addition biomass is a CO2 emissions neutral source of
energy. This is due to the fact that CO2 is taken from the atmosphere
and used by plants to grow. Planting trees and crops does soak up an amount
of carbon dioxide but it’s limited, if these are then harvested for biomass
fuels fossil fuel use is offset thus further reducing the emission of CO2.
Although some CO2 is emitted when manufacturing and burning biomass
fuels, it is ultimately equal to the carbon dioxide absorbed by the plants
used to produce this fuel (if the crops and trees are sustainably managed).
For example the use of short rotation coppice (SRC) of willow and poplar,
as a substitute for fossil fuels, produces no net CO2 emissions
and low emissions of nitrogen and sulphur pollutants from its combustion.
In this way fuel produced by energy crops could help ‘phase out’ fossil fuel
generation as it would be a carbon dioxide (CO2) neutral energy, providing
the rate of consumption is equal to the rate of re-planting. This neutrality
is another reason biomass is a very viable source of energy. The chart below
indicates the versatility of bioenergy.

Contrary
to what some might believe this type of forestry does not involve cutting
down rainforests and ancient woodlands. It is a technique initially used in
Sweden. Coniferous trees are planted at high density and after a period of
growth selective cutting reduces the density of trees. The thinning produces
wood chip that is later used for various purposes.

Short
rotational arable coppicing, is currently viewed by some as a potentially
important source of fuel for electricity generation in Scotland and the UK
- the UK Governments Department of Trade and Industry estimates (Energy Paper
62) that the maximum total realistic UK resource potential by 2025 could be
up to 150TWh/yr - half the current UK electricity demand! Some farmers already
burn straw in special plants used on their farms for power. Over the
last couple of years, there has been great debate over the future of energy
crops in Scotland: whether or not it makes sense to utilise agricultural
land, especially ‘set aside’ land to grow an energy source for the future.
Currently farmers get paid for ‘set aside’ land as part of government subsidies
and it appears to make far more sense to grow energy crops rather than pay
farmers for doing nothing.

Crops
tend to be established by planting cuttings in cultivated ground. Coppicing
occurs every 2-4 years when stems are harvested by cutting 5-10cms above ground
level (cutting cycle). Cutting cycles can vary depending on the objectives
of management and available land - research indicates that a yield increase
of up to 70% can be achieved for one 4-year rotation compared with two 2-year
rotations grown over the same periods.

Energy
crops include

· Willows

· Poplars

· Hemp

· Miscanthus: a temperate climate grass adapted to moist soils (sewage
sludge can be used as fertiliser for this plant)

· Maize

· Sorghum (a grass like grain crop that produces sugars)

Willow
has been the main tree species highlighted for SRC - poplar may also be used.
This is because willow is an ideal species to grow in the Northern hemisphere,
mainly in cold and wet areas. It produces large amounts of biomass in relatively
short periods due to its fast growing nature. It is reasonably simple to establish
and requires low input of agricultural chemicals during its growth. Willow
will thrive in a wide range of soil types and can be grown from cuttings provided
there is an adequate water supply (and as far as “sunny” Scotland is concerned
this does not pose a problem). Care needs to be taken to control weeds and
protect new plantations from rabbits and deer. During growth the use of fertilisers
and chemicals is lower than for most agricultural crops. The willow can be
cut back at the end of the first year as this encourages growth of multiple
shoots, alternatively plantations could be removed and the land returned to
agricultural crops if things are not going as planned. Yields from coppicing
can be stored until needed and then used to generate heat and electricity.

So
far SRC does not appear to carry much wait in terms of economic viability
- this is improving. Prices will have to fall in order to become a real option
for the future or the level of subsidies will have to increase in progress
is to be made.

Forestry
residues are produced when controlled thinning of plantations and trimming
of felled trees is undertaken to reduce forest fire risk and to accelerate
the forest growing rate that can sometimes be prevented if the area is overpopulated.
This waste is usually just left to rot on the forest ground; extracting and
collecting it clears up the forest making it easily accessible and manageable.
In this category waste from public gardens and woods can also be included.
The waste can be collected, dried and used as fuel.

These
include straw, manure, vegetables, fruit and general garden waste. Until recently
the excess straw produced in the UK was burned in the fields or ploughed back
into the land. As of the end of 1992 environmental legislation put in place
has restricted field burning and thus straw has been seen as a potential source
of energy. Other residues include potatoes and sugar beet tops as well as
damaged fruit and around 5 million tonnes of nursery wastes. Using agricultural
residues as a source for energy tackles another problem apart for the need
to find alternative energy sources. Agricultural residues include animal wastes.
Use of these wastes reduces the possibilities for odour and water pollution
by manures. Manures from cattle, chickens and pigs are the most common ‘wet
wastes’; in the UK about 7 million tonnes of such wastes are produced in a
year!

Various
plants have seeds that can be crushed on the farm to produce a range of vegetable
oils and although they are not good enough for human consumption they can
be used to power motors and onsite generators like those used for combined
heat and power plants (CHP).

One
of the main sustainable development issues in Scotland is the effective management
of waste. Waste disposal sites are a source of pollution, in terms of emissions
to the atmosphere and water and are associated with health and environmental
effects. Ultimately, the goal is to reduce waste production and to maximise
recycling and reuse. But the waste that is produced needs to be dealt with
effectively and sometimes energy generation is the optimum choice due to technical,
geographic or market barriers to recycling.

Energy
can be generated from the vast amounts of municipal and industrial waste that
society produces.

This
is waste that relates to a city or town, therefore it is wide ranging in composition,
for example:

· Paper and paper products

· Plastic

· Rubber and leather

· Textiles

· Wood

· Food wastes

· Yard wastes

· Glass and ceramics

· Metals

· Miscellaneous (including
even fridges!!)

This
can cause problems when the waste is incinerated as different elements burn
at different temperatures and speeds, leading to an uneven mixture and some
waste not being completely burned. This means that energy is not extracted
as efficiently as possible. Scotland creates 3 million tonnes per year of
MSW, 90% of which goes to landfill, 5% to recycling and reuse, and 5% to incineration
(DETR, SEPA). Incineration is therefore a small part of waste management for
Scotland when dealing with MSW. In the long term, this picture will change
and Scotland will follow the European example of countries like Denmark, where
at present 60% is recycled, 35% to EFW, and 4% to landfill. The EU Landfill
directive aims to divert wastes from landfills and will also influence the
increase in the number of incinerators. These will all be part of a grand
picture where recycling, reuse and EFW all work together to deliver an integrated,
sustainable waste strategy.

Waste
from timber processing is a great source of biomass feedstock. Dry sawdust
and offcuts usually thrown away after the processing o cut timber make exceptionally
good fuel. The furniture industry in the UK is estimated to produce 35,000
tonnes of such residues a year!

To
date, one of the key characteristics of fossil fuels are that they can be
easily acquired, transported and stored for use without their intrinsic energy
content being compromised. This means that in order for biomass to be a competitive
rival to fossil fuels it should be transportable and readily available for
use. Unfortunately this is not as easy as it may sound; biomass is wholly
organic and thus has a short shelf life by nature. For example the water content
of biomass does not contribute to its stored energy. Water contents can be
as high as 95% for fresh plants! This means that only 5% of the plant has
energy to be tapped into. Further more if the matter is not dried then decomposition
sets in quickly and renders the feedstock unusable. This means that in order
to use biomass it has to be dried to reach water content of about 20%.
In addition transporting biomass resources poses a problem i.e. they have
to be processed in such a manner that aids transportation. For example if
plant matter is dried and then chipped not only will burning it be made easier
but transporting will also be simple.

As
stated above biomass is a mixture of organic compounds and whatever form it
comes in it must be used within a short period of time on site (otherwise
it must be processed so that it’s shelf life is extended).

This
physical processing involves:

· Removing the moisture
(This can be achieved by in-situ drying facilities)

· Chipping or creating
fuel pellets (Chippers can be placed next to dryers to prepare the feedstock
for transportation, storage or immediate use).

This is a result of processing MSW, facilitating recycling, re-use and ensuring
the homogenous nature of the waste. The non-combustible elements of the waste
are left over after processing, giving the waste a higher calorific value.
The new improved composition of the waste allows increased efficiency from
incineration and reduced emissions and products as the waste that creates
the harmful effects can be removed during processing. Said processing involves

This
type of conversion allows for and otherwise very cumbersome fuel resources
to become easily transported and more hygienically handled, it also allows
incineration to play a part in waste management where reduction, reuse and
recycling can be maximised.

Further
processing has also been developed; the end product is called densified
refuse derived fuel (d-RDF). This is a process by which the combustible
part of the waste is separated, pulverised, compressed and dried to produce
solid fuel pellets about 5cm long.

In
order to produce bioenergy various processes can be used to convert the intrinsic
chemical energy of biomass directly to heat or electricity or to the intermediate
biofuel. Biofuels include Methane Gas, Liquid Ethanol and Methanol
or Solid Char or Charcoal.

All
fuels contain two combustible constituents; the volatile matter and
char. As the temperature of the fuel rises the volatile matter is released
in the form of vapours or vaporised tars and oils. The spurts of flame, for
example as wood burns, are an indication of the combustion of these products.

After
this process (which is known as thermal degradation) ends the solid remnants
comprise of char and inert matter. The char, which is mainly carbon, can be
further combusted to produce heat and CO2. The inert matter then
becomes clinker, slag or ashes.

Most
of the bioenergy is within the initial volatile matter. This means that any
furnace designed to burn biomass fuels should be designed in such a way to
ensure complete combustion of these vapours. In addition, air must reach all
of the char; this could be accomplished if small pieces of the matter are
burnt (another reason behind the need to physically process biomass before
it’s use)

The
following diagram illustrates how combustion can be used to produce energy:

Mass
Burn Combustion (MBC) can use municipal Solid Waste (MSW) to generate electricity.
This is one of the main methods of incineration and has been around for years.
It is commercially available and has been optimised. The plant operates by
feeding waste onto a moving grate where it is burned; the heat generated by
this is used to generate steam that drives a generator to produce electricity.
The burning of the waste produces two types of ash. Incinerator Bottom ash
falls through the grate for collection and is either landfilled or used in
the construction industry. Fly Ash, which escapes with the flue gases that
are emitted to the atmosphere, can contain sufficient dioxins and metals that
require cleaning. As it involves the mass burning of MSW; this has many concerns
attached to it.

In
particular the waste is a mixture of different materials if these are combusted
they may have a detrimental affect on the environment. The main pollutants
that result from MSW incineration are as follows:

· Gases

· Metals

· Organic Substances

· Particulate Matter

The
main area of concern is the contents of the array of gases that are emitted
from the plants. The gases contain dioxins, which are suspected of causing
many health problems including cancer. The particulate matter is also an area
of concern, focusing particularly on ultra fine particles less than ten millionths
of a metre. These are often inorganic materials with metals and organic compounds
on their surface. In Scotland, all releases to the environment are regulated
by SEPA, the Scottish Environmental Protection Agency.

This
method of incineration uses RDF and is an alternative to the mass burn system.
The pellets are fed onto a bed consisting of a mixture of sand and dolomite
mineral. Air is then pumped through the whole mixture to create a bubbling
liquid. The waste in this new liquid form has an improved combustion efficiency
that reduces pollution and increases generation per ton of waste.

A
downfall of this technology is that it is slower than MBC and there is limited
experience. This form of incineration though has not yet been proven on a
commercial scale and requires further investigation.

Two
techniques that are very promising for the future of waste incineration are
Gasification and Pyrolysis. These technologies are not as developed as MBC
but promise many benefits. The two techniques have very similar economic characteristics,
there is an option of pressurising the gas, which increases the capital costs
but is compensated by cost savings at generation. The savings are created
as compression of the gas is no longer required and there is higher system
efficiency.

The
gasification process in general involves the reaction of a solid fuel with
hot steam and air (or oxygen) and the subsequent production of a gaseous fuel
by partial oxidation. The diagram bellow illustrates the process. Gasifiers,
depending on their type can operate with temperatures varying from a few hundred
to over a thousand degrees Celsius and from pressures from around atmospheric
(1 atmosphere) up to 30 atmospheres.

· The gas resulting
from this process mainly consists of:

· Carbon monoxide (CO)

· Hydrogen (H)

· Methane (CH4)

· Carbon dioxide (CO2)

· Nitrogen (N) The
proportion of the gases in the mixture depends on the processing conditions
and whether air (78% Nitrogen, 20% Oxygen and 2% of others) or oxygen was
used.

The
simplest of gasification processes result in gases containing up to 50% by
volume of CO2 and N. This means that the fuel has a low energy
value so transporting is not economical viable but on site use can prove to
be beneficial. There are benefits to using such a complicated process
to produce energy from biomass. The resulting gas is cleaner and more versatile
than the original biomass; any unwanted pollutants can be removed during processing.

Gasification
using oxygen instead of air produces a mixture of gases containing Hydrogen,
Carbon Monoxide and Carbon Dioxide. Removing the CO2 produces a
mixture called Synthesis Gas, this gas can then be used to produce
almost any hydrocarbon. The most common products are methane and methanol.
Methane is a combustible gas that can be used to drive generators although
it is a very dangerous and harmful greenhouse gas. Methanol is a liquid fuel
that is a direct substitute for gasoline

This
age old process, otherwise called destructive distillation, involves
the heating of the original biomass in the near absence of air, the temperatures
at which this occurs range from 300 to 500 degrees Celsius. These high temperatures
drive the volatile matter out of the original material, what is left is the
char (charcoal). The usual biomass material used for pyrolysis is wood but
nutshells and MSW can be used as well.

Further
technological advancement in this sector has lead to a process called fast
pyrolysis. This involves the collection of the volatile matter and depending
on the temperature of the process these materials can be combusted. This liquid
product has the potential to be used as fuel oil. Temperatures range from
800 to 900 degrees Celsius. Fast pyrolysis can leave as little as 10% char
and can convert as much as 60% into a gas.

Gasification
and Pyrolysis have very similar costs but these are again hard to measure
for the sake of generalisation or comparison as they are site specific and
there are no large plants to use as an example. They do promise improved efficiencies
and lower environmental mitigation costs with relevance to MBC. Furthermore
if pressurised gas is used to drive a turbine then this increases capital
costs but leads to a saving in terms of generation due to higher system efficiency.
They allow improved combustion due to the intermediate fuel that is produced
and have lower emissions due to lower gas flows. The production of this fuel
means that it can be transported for generation at a different site. To further
the technology and to make it economically viable, more funding for research
and development is necessary, as well as the financing of the first large-scale
commercial plant.

Anaerobic
digestion occurs in the absence of air, the decomposition in this case is
caused not by heat but by bacterial action. Any organic substance can become
subject to anaerobic digestion so long as there are warm, wet and airless
conditions. For example ‘marsh gas’ is a product of the anaerobic digestion
of vegetation at the bottom of ponds, this gas rises to the surface and bubbles,
it is also combustible. With the aid of human intervention there are two products
of this process, biogas and landfill gas. The chemical processes behind the
production of these gases are very complex. The figure below shows the generalised
process.

Biogas
is generated from concentrations of sewage or manure. These are usually in
the form of slurry comprised mostly of water (almost 95%). The slurry is fed
into a digester, this input can be continuous (usually the case with sewage)
or in batches. The digestion continues from about ten days up to weeks. The
temperature in the digester should be kept at 35°C and although the digestion
itself produces heat, in colder climates some top up heat should be provided.
In order for the process to remain sustainable the excess heat should be provided
by the biogas itself. In the very extreme cases all the produced biogas
has to be used for this heating. In these cases the process is still beneficial
as it offsets the need to use fossil fuels in order to process the wastes.

As
already stated a huge proportion of the waste produced in Scotland goes to
landfill sites. As this waste sits under the ground in these sites, the biodegradable
organic matter within the waste goes through anaerobic decomposition and produces
a gas that is roughly an even mixture of Carbon Dioxide and Methane. This
is an explosive mixture and has been known to cause explosions under ground.
This gas used to be flared off or released to the atmosphere. The combustion
of this gas reduces net emissions of carbon dioxide if used to offset generation
from fossil plants as less is produced. This represents a small resource,
but it is economically competitive with other forms of generation and it can
provide base load electricity output.

The
length of time and amount of gas that is available from a landfill site is
very specific to the type of waste, moisture content, temperature, acidity
and the design of the site. As the diagram above shows, gas is drawn up from
vertical or horizontal wells through a system of pipes. At this stage the
gas is usually warm and saturated with moisture. The extraction pipes contain
condensate traps and are laid at an angle so that as the gas cools, the moisture
does not hinder the flow of the gas. The condition of the gas is specific
to what use it will be put to. The plant is constructed so that there is no
leak of gas to the surrounding land or air. This methane and CO2
mixture can be used in the same combustion process discussed previously. The
generation equipment is usually contained within the same area as the extraction
plant. This site is usually away from urban sites due to safety reasons and
amenity.

Fermentation
is also an anaerobic process. With this process Sugars with the use of micro
organisms (usually yeast) are converted into ethanol. Ethanol can be used
as the fuel in the combustion processes. This can either be achieved through
mixing the ethanol with gasoline or by using it directly in some modified
combustion engines. Sugar cane undergoes fermentation most efficiently. Other
feedstock’s can be used such as potatoes and corn, but these require processing
so that the starch can be converted to sugar.

The
best way to sum up the various uses of Biomass and the processes involved
in its harnessing is to sum up the products that can be created from the feedstock
and to illustrate there uses (see figure below). So from all of the available
biomass feedstock that can be harnessed and through the various processes
the products include:

· Heat (that can be
used to heat water for use or central heating and to produce steam for use
with steam turbines)

This
technology is still in its infancy, therefore there are many unanswered questions
regarding its economic viability. The only way for this to be investigated is
through demonstration plants but these are expensive and require large investment.
Companies are not willing to invest in something that has a long or limited
pay back; this is where governmental support is key to the growth of the industry.

Which
biomass technology is the most economically viable depends on site-specific
circumstances. This depicts the type of feedstock that is available and therefore
which method of generation is best suited. The transportation of the feedstock
has the possibility to incur costs so obviously it makes sense to position plants
where minimum transportation is necessary.

There
are many beneficial factors to consider. Biomass feedstock production, handling
and processing due to the nature of all the materials are practiced in rural
areas and so these benefits would be for those areas. These include:

· Rural development through

· Regional economic gain

· Return of investments

· Employment opportunities

· Job creation

· Sustainability

But
on the other hand, there are many disadvantages to be considered. There will
be increased traffic flow in the area and the various plants will represent
a visual intrusion. Furthermore the running of the plants will create noise
that may be unacceptable for nearby residents. The plants may even affect local
ecology and any by-products and wastes must be removed thus adding to the traffic.
Therefore reducing the distance the products travel to their point of use is
an important part of this sustainable strategy. Only products requiring a specific
form of management should be transported and where possible this should be by
rail as residents are not keen on masses of lorries passing in front of their
otherwise peaceful landscape.

The
emissions from plants that use combustion are always a concern for local residents
or anyone who is affected by them. The gases and particulates are often carried
by the wind to another area other than that where generation takes place. This
poses problems and political issues which need to be addressed. The possible
detrimental effect on human health has the potential to rule out the use of
such plants, especially when there are other alternatives for generation.

Biomass
is by no means the only solution to global warming and other problems and the
environmental effects of its use should be examined very closely.

For
instance the combustion of wastes solves a disposal problem and may offset the
use of fossil fuels but the incombustible materials such as ash have to be removed
from the site. This incurs a cost to the environment due to pollution from transportation.
In addition if care is not taken and a conventional combustion chamber is used
(as opposed to a newly designed one that is capable of filtering out harmful
emissions) by-products such as particulates and poly-aromatic hydrocarbons (PAHs)
can escape to the atmosphere.

On
the other hand tree planting on a very large scale (such as the one needed for
arable coppicing) helps in the absorption of CO2. If care is taken
in the overall processing (harvesting, chipping, transporting, drying and so
on) of this biomass feedstock then the net CO2 emissions after it
has been used for fuel will be less then the absorbed CO2 thus benefiting
the environment.

In
defence of biomass use comes methane (!). Methane is a very harmful greenhouse
gas as well as causing accidental explosions due to its migration from landfill
sites to nearby buildings. Moreover it has 30 times the damaging effect on the
environment than that of CO2. The extraction, use of and combustion
of methane actually protects against global warming. Even though a by-product
of the process is carbon dioxide the net effect to the environment is much smaller.

Finally
another issue concerning the environment is the use of ‘set aside’ land
for energy crops. SRC has potential as there are only a few stumbling points
that may cause some objections; for example the potentially sensitive nature
of sites has to be considered - proximity to settlements and roads- as views
could be impeded due to the growth rates causing a 3-dimensional woodland type
mass in the field. But integrating the area with the surrounding landscape -
trees and other features- when establishing new plantations can reduce visual
impact. Also changes in the landscape can be quick during growth and also during
harvesting. Additionally the scale of SRC has to be considered to avoid saturation
of the landscape by monotonous planting. If farming conforms with regulations
about adjacent plots of different tree species and so on most of the above concerns
can turn out to be unfounded.

Biomass
has the potential to become an important part of electricity generation in Scotland.
It can be predicted and planned, therefore making it suitable for base load
generation. The technology, however, is not yet far enough advanced but as more
demonstration plants begin operation the industry will learn. At present, the
main focus for this technology is small scale plants for generating electricity
and producing heat. The economic support of the government will be key to the
realisation of these plants on a commercial scale.